Arlington, Va.—Conventional wisdom says a river's flood plain builds bit by bit, flood after flood, whenever the stream overflows its banks and deposits new sediment on the flood plain. But for some vast waterways in South America's Amazon River basin, that wisdom doesn't hold water, according to scientists funded by the National Science Foundation (NSF). Results of their research are published in the October 2nd issue of the journal Nature.
Seasonal rains wash billions of tons of rock and soil from the Andes Mountains each year. But new evidence suggests that it's only about once every eight years, when the equatorial climate phenomenon known as La Niña is in full swing, that water rises rapidly enough to move huge amounts of that sediment to the flood plains.
Rolf Aalto, a University of Washington (UW) geologist and lead author of the Nature paper, uses a lightweight tube-like device to extract core samples of such sediment deposits across the remote, thickly forested and unspoiled flood plains of northern Bolivia. He measures the amount and activity of lead-210, a radioactive isotope produced by the decay of naturally occurring radon. The isotope is deposited on the flood plain by fresh river sediment and by rainfall, and it can be used to date successive layers of sediment, for as long as a century after it is deposited, as the lead-210 decays further.
But when Aalto examined samples from the Beni River, which flows northward from the Andes into a Mississippi-sized Amazon tributary called the Rio Madeira, he was puzzled.
"I wasn't seeing a signal that looked anything like what I expected from previous studies on other river flood plains. I was befuddled," he said. "I would see a lot of lead-210 on the surface, and then the activity would drop down to an unchanging level of activity for a long way down, until the next big decrease."
The multitude of thin layers and gradually declining lead-210 activity he expected to see simply didn't show up. The reason: A few thick sediment layers were filling most of the core samples. Aalto devised a new dating method to better reflect a more complex flood plain environment, then analyzed new samples that showed similar large deposits at various intervals going back nearly 100 years. Each of the large layers was deposited at a time that correlated with a La Niña event in the equatorial Pacific Ocean.
La Niña is the cold phase of the El Niño Southern Oscillation climate phenomenon that now is recognized as having wide affects on weather throughout the Western Hemisphere.
"The whole system is being raised," Aalto said. Sediment is moved and deposited throughout the river system in episodes orchestrated by the El Niño cycle."
Thousands of years ago, the Beni flowed into the Mamore' near the Andes. But its course has changed many times through the millennia and the main channel now is hundreds of miles away from the Mamore'. Major floods in which a river shifts course don't happen often on human time scales, but these Bolivian rivers still can migrate large distances during a single flood, he said.
Aalto believes such active channel migration, periodic large sediment deposits on the flood plain and large floods might all be part of the general mechanics of large rivers. However, that theory cannot be fully explored in the United States because few rivers of appreciable size still function naturally. He hopes that studying largely unspoiled rivers such as the Beni and Mamoré will help to build a better understanding of how rivers behave in completely natural conditions, and that understanding can be applied to disturbed river systems in the United States and elsewhere.
"Rivers are the arteries of continents," said Aalto. "And the Beni is about as close to the heart of the Amazon as you can get."
Other authors of the Nature paper are Laurence Maurice-Bourgoin and Jean-Loup Guyot of Institut de Recherche pour le Développement (IRD) in France and Brazil, who have conducted extensive field studies in Bolivia and Brazil; Thomas Dunne of the University of California, Santa Barbara; and David Montgomery and Charles Nittrouer, both scientists at UW.
The research is funded by NSF and IRD.
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